Method for fabrication of a luminescent structure

ABSTRACT

A method for fabricating an LED/phosphor structure is described where an array of blue light emitting diode (LED) dies are mounted on a submount wafer. A phosphor powder is mixed with an organic polymer binder, such as an acrylate or nitrocellulose. The liquid or paste mixture is then deposited over the LED dies or other substrate as a substantially uniform layer. The organic binder is then removed by being burned away in air, or being subject to an O 2  plasma process, or dissolved, leaving a porous layer of phosphor grains sintered together. The porous phosphor layer is impregnated with a sol-gel (e.g., a sol-gel of TEOS or MTMS) or liquid glass (e.g., sodium silicate or potassium silicate), also known as water glass, which saturates the porous structure. The structure is then heated to cure the inorganic glass binder, leaving a robust glass binder that resists yellowing, among other desirable properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/385,510 filed on Sep. 16, 2014, titled “Phosphor inInorganic Binder for LED Applications”, which is a §371 application ofInternational Application No. PCT/IB2013/052549 filed on Mar. 29, 2013,which claims priority to U.S. Provisional Patent Application No.61/617,076, filed Mar. 29, 2012. Ser. No. 14/385,510, PCT/IB2013/052549,and 61/617,076 are incorporated herein.

FIELD OF THE INVENTION

This invention relates to a phosphor layer for use with light emittingdiodes (LEDs) to wavelength convert the LED emission and, in particular,to a technique for forming the phosphor layer with an inorganic glassbinder for improved performance.

BACKGROUND

Providing a phosphor layer over an LED is common. Typically, a phosphoris provided over a blue LED to make white light. Blue light leakingthrough the phosphor, combined with the phosphor light, produces whitelight. There are many ways to provide the phosphor layer over the LED,and one prior art technique is described below.

It is known to pre-form a layer of phosphor powder mixed with siliconeand then laminate the layer over blue-emission LED dies mounted on asubmount wafer. The wafer is then singulated. The resulting dies emitwhite light. This is described in United States patent applicationpublications 20110031516 and 20110266569, by Grigoriy Basin et al.,assigned to the present assignee and incorporated by reference. Othertechniques also mix the phosphor powder in an organic polymer binder(e.g., silicone or epoxy) and then deposit (e.g., print, mold, etc.) theliquid/paste layer directly over the LEDs. The binder is then cured toharden it.

However, the heat and high flux from the LEDs tend to oxidize theorganic binder surrounding the phosphor particles, causing the binder toyellow and color shift the light. Further, high quality silicone andepoxy are relatively expensive, which is a significant concern for largeremote phosphor components.

What is needed in a process for forming a phosphor layer that can beeither formed directly over LEDs or formed on a transparent substrateand which does not use silicone or other organic polymer as a binder.

Although inorganic glass would be a relatively stable and reliablebinder for the phosphor powder, molten glass is too chemically reactiveat the high temperatures needed to form a phosphor-glass layer, sincethe glass would react chemically with the phosphors, especially rednitride phosphors.

A sol-gel for forming a glass layer may also be considered as acandidate to substitute for the silicone, but the sol-gel is also tooreactive for the phosphor, leading to light attenuation. Further, thelow viscosity of sol-gel would result in phosphor sedimentation andnon-uniform phosphor density. Other problems would exist as well.

SUMMARY

A phosphor layer is formed, having an inorganic glass binder, that canbe used as a remote phosphor for LEDs or as a coating over LEDs towavelength-convert the LED light. In one embodiment, the LED emits bluelight, and the phosphor converts the light to white light.

Initially, phosphor powder is mixed with an organic polymer binder, suchas inexpensive acrylate or nitrocellulose. The mixture may be formed asa paste having a wide range of viscosities. Such a mixture is theneasily screen printed over a transparent substrate, such as a thin glasssubstrate. Alternatively, the mixture can be screen printed over LEDdies mounted on a submount wafer. Other deposition techniques may beused. The layer may be made to have an accurate thickness, such as+/−2%.

The organic binder is then burned away in air, such as at 180-300degrees C. Alternatively, the mixture may be subject to an O2 plasmaprocess. In either case, the polymer is oxidized and evaporates. Thebinder may also be chemically dissolved.

The resulting layer is a porous, sintered phosphor powder layer that issubstantially uniformly distributed over the transparent substrate orLEDs. The porous layer is relatively weak and subject to contamination.

Next, the porous layer is impregnated with a sol-gel (e.g., a sol-gel ofTEOS or MTMS) or liquid glass (e.g., sodium silicate or potassiumsilicate), also known as water glass, which saturates the porousstructure.

The structure is then heated to cure the inorganic glass binder. In thecase of sol-gel, the heating causes the sol-gel to become cross-linkedto create a hard, scratch resistant glass layer. In the case of liquidglass, the water component evaporates, leaving a hard layer. Otherglassy materials may be used.

The resulting inorganic binder material is extremely stable under thehigh heat and flux generated by the LEDs, it resists yellowing, itconducts heat much better than silicone or epoxy, it is much lessexpensive than silicone or epoxy, and it has greater mechanical strengthand scratch resistance.

Other embodiments are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art blue or UV flip-chip LEDdie, mounted on a submount.

FIG. 2 is a top down view illustrating a simplified submount waferpopulated by an array of LED dies, such as 500-4000 LEDs, where all LEDdies on the wafer are simultaneously processed.

FIG. 3 illustrates the submount wafer of FIG. 2 having a phosphor layerbeing laminated or deposited over the LEDs.

FIG. 4 illustrates the submount wafer of FIG. 2, populated with LEDsencapsulated with a lens, having a phosphor layer being laminated ordeposited over the lenses.

FIG. 5 is a cross-sectional view of the phosphor layer deposited on asubstrate, where phosphor powder is distributed in a polymer binder. Thesubstrate may be a glass substrate or the LED dies on the submountwafer.

FIG. 6 illustrates the phosphor layer of FIG. 5 after the polymer binderhas evaporated by oxidation in an O2 plasma or baked out in air, leavinga porous phosphor layer.

FIG. 7 illustrates a sol-gel glass, liquid glass, or other suitabletransparent inorganic binder material saturating the porous phosphorlayer, then being cured to form a robust inorganic binder.

FIG. 8 illustrates the phosphor layer of FIG. 5 being laminated over asubmount wafer populated with LED dies.

FIG. 9 illustrates the phosphor layer of FIG. 8 conforming to the shapeof the LED dies upon application of a vacuum or other form of pressure.

FIG. 10 is a cross-sectional view of a completed LED having a phosphorlayer over it formed in accordance with one embodiment of the presentinvention, where the phosphor layer is formed over the entire surface ofthe submount wafer prior to singulation.

FIG. 11 is a cross-sectional view of a completed LED having a phosphorlayer over it formed in accordance with another embodiment of thepresent invention, where the phosphor layer is affixed as a tile overthe top of an LED die.

FIG. 12 is a flowchart summarizing techniques for fabricating thephosphor layer in accordance with some embodiments of the invention.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION

Although the present invention of a phosphor layer does not rely on itsuse with any particular LED, an example of an LED structure will bedescribed to illustrate the phosphor layer's use with an LED.

Prior art FIG. 1 illustrates a conventional flip chip LED die 10 mountedon a portion of a submount wafer 12. In a flip-chip, both the n and pcontacts are formed on the same side of the LED die.

In this disclosure, the term “submount wafer” is intended to mean asupport for an array of LED dies, where electrical contacts on the waferare bonded to electrodes on the LED dies, and the wafer is latersingulated to form one or more LEDs on a single submount, where thesubmount has electrodes that are to be connected to a power supply.

The LED die 10 is formed of semiconductor epitaxial layers, including ann-layer 14, an active layer 15, and a p-layer 16, grown on a growthsubstrate, such as a sapphire substrate. The growth substrate has beenremoved in FIG. 1 by laser lift-off, etching, grinding, or by othertechniques. In one example, the epitaxial layers are GaN based, and theactive layer 15 emits blue light. LED dies that emit UV light are alsoapplicable to the present invention.

A metal electrode 18 electrically contacts the p-layer 16, and a metalelectrode 20 electrically contacts the n-layer 14. In one example, theelectrodes 18 and 20 are gold pads that are ultrasonically welded toanode and cathode metal pads 22 and 24 on a ceramic submount wafer 12.The submount wafer 12 has conductive vias 24 leading to bottom metalpads 26 and 28 for bonding to a printed circuit board. Many LEDs aremounted on the submount wafer 12 and will be later singulated to formindividual LEDs/submounts.

Further details of LEDs can be found in the assignee's U.S. Pat. Nos.6,649,440 and 6,274,399, and U.S. Patent Publications US 2006/0281203 A1and 2005/0269582 A1, all incorporated herein by reference.

FIG. 2 is a simplified top down view of an exemplary submount wafer 12on which is mounted an array of LED dies 10 (only one LED is numberedbut all of the squares on wafer 12 are LEDs). There may be 500-4000 LEDson a single submount wafer 12. All LEDs on the wafer 12 will beprocessed simultaneously using the method described below.

FIG. 3 illustrates a portion of the submount wafer 12 having LED dies 10mounted thereon. The LED electrodes 18 and 20 are bonded to metal padson the submount wafer 12 that are connected to more robust pads on thebottom surface of the wafer 12 by vias extending through the wafer 12.When the wafer 12 is later singulated, the bottom pads may be solderedto pads on a printed circuit board.

A phosphor layer 38 is shown over the wafer 12, which represents thateither the phosphor layer 38 will be laminated over the wafer 12 or thatthe phosphor layer 38 will be a remote layer separated from the LED dies10. In another embodiment, the phosphor layer 38 may be singulated toform tiles, and each tile is affixed over an LED die 10. Further, inanother embodiment, the phosphor layer 38 is laminated over an LED waferprior to the LEDs being singulated. In that way, there is little wasteof phosphor. Other uses of the phosphor layer 38 are envisioned.

FIG. 4 illustrates how the LED dies 10 may have a hemispherical lenses36 molded over them to separate the phosphor layer 38 from the dies 10after lamination. This reduces the intensity of heat and flux on thephosphor and improves color uniformity vs. angle.

FIGS. 5-7 illustrate the formation of a phosphor layer in accordancewith one embodiment of the invention.

In FIG. 5, a substrate 40 may be a transparent glass plate, or thesurface of the submount wafer 12 populated with the LED dies 10, or anyother suitable substrate. If the phosphor layer 38 (FIGS. 3 and 4) isintended to be a self-supporting layer for a remote phosphorapplication, or is intended to be singulated as tiles and affixed on LEDdies 10, then a glass substrate is preferred. A remote phosphor,supported by a transparent glass plate and separated from the LED(s),may be preferred for high power applications using an array ofinterconnected LEDs, where the light and heat generated are very high,or in situations where the emitted color is desired to be selectable byselecting to use one of a variety of remote phosphor plates havingdifferent characteristics. If the phosphor layer 38 is intended to belaminated directly over the LEDs on submount wafer 12 or over an LEDwafer, then the substrate 40 would be the submount wafer 12 populatedwith the LED dies 10 or an LED wafer.

Initially, phosphor powder 42 is mixed with an organic polymer binder44, such as an acrylate, nitrocellulose, or other commercially availablebinder that can be later evaporated or dissolved away. Due to theconventional small grain size of the phosphor powder 42 and its densityin the binder 44, the distribution of phosphor powder 42 in binder 44 isfairly homogenous which should achieve good color uniformity across thephosphor layer 38. The phosphor grain size and density is not critical;however, the density should be such that the grains contact each otherafter the binder 44 is later removed.

The phosphor powder 42 may be any conventional phosphor, including YAG,red, green, yellow, blue, or orange phosphor, or combinations thereof,depending on the application and the LED.

Since the polymer binder 44 does not have to have good opticalproperties, it may be much less expensive than silicone. Silicone foruse with phosphors is about $900 US/kg.

The mixed binder/phosphor may form a paste or viscous liquid that isspray coated, dip coated, spun on, or screen printed onto the substrate40. The thickness is not critical for the invention, but the thicknessand density of the phosphor powder 42 affects the emitted light color.The deposited layer should have a thickness accuracy of +/−2% to achievegood color uniformity.

In FIG. 6, the binder 44 is subjected to heat, such as 180-300 degreesC. in an air environment for a time to evaporate the binder 44.Alternatively, an O2 plasma may be used to oxidize the binder 40 toconvert the binder 44 to a gas, or the binder may be chemicallydissolved and heated. The combination of heat and the removal of thebinder 44 results in the phosphor powder 42 grains being sinteredtogether and to the substrate 40 to form a relatively uniform layer ofphosphor powder 42. The phosphor powder 42 forms a porous layer or aweb.

In FIG. 7, the porous layer is then impregnated and saturated with asol-gel (for forming a glass) or liquid glass, also known as waterglass. This may be done by spray coating, dip coating, or otherwell-known method.

After the saturation step, the liquid glass or sol-gel is cured todehydrate the material, leaving a robust glass binder 48 surrounding thephosphor powder 42. In some cases, curing is performed by heat ordehydrating at a longer time at room temperature, or performed using achemical curing agent.

Liquid glass (sodium silicate or potassium silicate) is made by fusingvarying portions of sand (SiO2) and soda ash (Na2CO3), where CO2 isdriven off. The ratio of these portions determines the properties of thefinal product. This product is specified as a ratio of SiO2/Na2O and asa concentration in water. The sodium may also be replaced by potassiumor lithium in order to obtain different properties. After applyingliquid glass as a thin film, the water is evaporated, leaving a solidglass coating behind. Lower SiO2/Na2O ratios tend to retain water betterand hence evaporate slower. Higher ratio solutions (approx. 2.8-3.22)are preferred if increased durability is desired. Complete dehydrationtypically requires heat during the drying process. The silicate layersmay be cured at a temperature of 250° C., which is well below thetemperature that luminescent materials can stand (nitride-basedluminescent materials can stand temperatures up to 350° C. and YAG-basedluminescent materials even much higher).

Another method to make the coatings durable is to make use of chemicalsetting. Chemical setting agents that can be used in this manner includemineral and organic acids, CO2 gas, and acid salts such as sodiumbicarbonate.

When silicate films are completely dehydrated, they provide excellentresistance to high temperatures. Most silicates have flow points around850° C. In LEDs, such temperatures will never be reached.

Liquid glass is transparent for visible light, and the transmissiondrops off rapidly below 400 nm, exhibiting a value of approximately 40%at 325 nm. For LEDs that convert blue to white light, this range issufficient.

Silicate coatings may be brittle. If a higher degree of flexibility isrequired, typically 5% by weight of glycerine can be added. Glycerinehas a very high transparency for blue light. Other materials may beadded, such as ethyelene glycol, propylene glycol, an alcohol, etc.

There are many suitable sol-gel materials, such as TEOS(tetraethylorthosilicate), MTMS (methyltrimethoxysilane), and MTES(triethoxysilane), all generally referred to as glass materials. Sol-gelis relatively inexpensive (less than $20 US/kg), so the resultingphosphor layer is less expensive than phosphor powder in a siliconebinder. Glass has high thermo and photo-thermal stability and resistsyellowing in the presence of the high heat and flux of an LED. Forming asol-gel of such materials is well known for depositing the materials ona substrate.

The sol-gel process is a wet-chemical technique commonly used for thefabrication of a glassy coating. In this process, the sol (or solution)evolves gradually towards the formation of a gel-like network containingboth a liquid phase and a solid phase. The micron-size orsub-micron-size glass particles become linked, forming the gel. Theformation of a TEOS, MTMS, MTES, or other glass layer using sol-gel iswell-known and need not be described in detail.

The drying process serves to remove the liquid phase from the gel,yielding an amorphous glass (a linked silica network or matrix).Subsequent thermal treatment (firing) may be performed in order todensify the glass to enhance its mechanical properties.

Since there will be shrinkage after dehydration, the thickness of thesol-gel or liquid glass should be adjusted to ensure complete coverageof the phosphor powder 42 after dehydration. Multiple applications andcuring of the sol-gel or liquid glass may be desirable for completecoverage.

The glass layer may be formed to have a relatively high index ofrefraction, comparable to that of high index silicones, to provide goodlight extraction.

As an alternative to depositing the film of FIG. 5 directly over the LEDdies 10 or depositing the film on a glass substrate, the film may bepre-formed as a flexible layer then laminated over the LED dies 10 andsubmount wafer 12, as shown in FIGS. 8 and 9.

In one embodiment, a flexible layer of the polymer binder 44 andphosphor power 42 mixture is formed on a releasable film (an embodimentof substrate 40). The layer may be tested to determine its luminescentproperties and matched to a particular bin of LED dies that emit aparticular narrow range of blue light. As shown in FIG. 8, the flexiblelayer 58 is then laminated on the matched LED dies 10 with thereleasable film facing up. A vacuum or mechanical downward pressure isused to ensure there are no air gaps and to conform the layer 58 to theLED dies 10. The releasable film is removed before or after the layer 58is fully laminated over the LED dies 10. Then, as shown in FIG. 9, theprocesses of FIGS. 6 and 7 are performed to create a robust inorganicglass layer 59 containing the phosphor powder 42.

FIG. 10 illustrates the structure of FIG. 9 (using a laminated layer 58)after hemispherical lenses 60 are molded over the LED dies 10 and afterthe submount wafer 12 is singulated. The structure may emit white lightor any color of light. The structure of FIG. 10 may also be formed bydepositing the layer 44/42 of FIG. 5 directly over the LED dies 10 byspray coating, spin coating, etc., then performing the processes ofFIGS. 6 and 7.

FIG. 11 illustrates an LED structure where the luminescent structure ofFIG. 7 has been singulated to form luminescent tiles 40/42/48, and eachtile is affixed over an LED die. In one embodiment, the LED die has aYAG tile 64 affixed directly over the top surface of the die, and aluminescent tile 40/42/48 containing red phosphor powder, formed usingthe processes of FIGS. 5-7, is affixed over the YAG tile 64 to create awarmer white light. A thin glass layer, epoxy, or silicone may be usedas the adhesive.

FIG. 12 is a flowchart summarizing various steps for forming thephosphor layer in accordance with some embodiments of the invention.

In step 70, an LED wafer is fabricated, such as a wafer containingGaN-based blue LEDs.

In step 71, the LED wafer is diced, and a submount wafer is populatedwith the LED dies.

In step 72, a mixture of phosphor powder and an organic binder iscreated as a paste or a liquid for screen printing or other type ofdeposition, or created as a lamination layer.

In step 73, the phosphor/binder mixture is printed or laminated on aglass plate (e.g., for a remote phosphor) or other substrate or over theLED dies.

In step 74, the organic binder is removed by heating or subjecting thebinder to an O2 plasma or other treatment, leaving a porous, sinteredphosphor powder layer.

In step 75, the phosphor “web” is impregnated with liquid glass, asol-gel, or other suitable material by spray coating, dip coating, orother process.

In step 76, the liquid glass or sol-gel is cured such as by heating tocross-link the sol-gel layer and/or evaporate any solution, such aswater in the liquid glass.

Steps 75 and 76 may be performed multiple times due to shrinkage.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A method for fabricating a luminescent structurecomprising: a. depositing phosphor layer on a surface, the phosphorlayer comprising phosphor powder mixed with a first binder; b. removingthe first binder and heating the phosphor layer, leaving a porousphosphor layer on the surface comprising phosphor grains sinteredtogether; c. infusing the porous phosphor layer with a sol-gel or liquidglass; and d. curing the sol-gel or liquid glass to form an inorganictransparent glass binder.
 2. The method of claim 1 wherein the step ofremoving the first binder comprises heating the first binder toevaporate the first binder.
 3. The method of claim 1 wherein the firstbinder comprises nitrocellulose.
 4. The method of claim 1 where the stepof depositing the phosphor layer comprises depositing the phosphorpowder and first binder on a transparent support substrate, the methodfurther comprising singulating the transparent support structure to formluminescent tiles and affixing one of the tiles over a light emittingdiode (LED) die.
 5. The method of claim 4 wherein affixing one of thetiles over the LED die comprises affixing one of the tiles over anotherphosphor tile affixed over the LED die.
 6. The method of claim 1 whereinthe sol-gel or liquid glass, after curing, forms a layer of inorganicglass.
 7. The method of claim 1 wherein the sol-gel or liquid glass,after curing, forms a layer of either TEOS, MTMS, or MTES.
 8. The methodof claim 1 where the step of depositing the phosphor layer comprisesdepositing the phosphor powder and first binder on a transparent supportsubstrate, wherein the support structure is a transparent lens between alight emitting diode die and the phosphor layer.
 9. The method of claim1 wherein the step of depositing the phosphor layer comprises depositingthe phosphor layer over a light emitting diode die.
 10. The method ofclaim 1 where the step of depositing the phosphor layer comprisesdepositing the phosphor powder and first binder on a transparent supportsubstrate, the method further comprising positioning the porous phosphorlayer, infused with the glass binder, and the support structure over alight emitting diode (LED) die so the phosphor layer is not in directcontact with the LED die.